Cold worked aluminum base alloy and method of producing it



Patented June 10, 1941 COLD'WORKED ALUMINUM BASE ALLOY AND METHOD OF PRODUCING IT v Philip T. ,Stroup, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania.

N Drawing.

Application August .23, 1939,

Serial No. 291,544

4 Claims.

My invention relates to improving wrought aluminum base alloys containing magnesium.

Copper-free aluminum base alloys containing a substantial amount of magnesium have been found to possess many technically advantageous properties both in the cast and wrought forms.

More particularly, because of the strength and resistance to corrosion which these alloys exhibit, they have become popular for certain uses, and could be more widely employed if their adaptability to certain working processes could be improved. In respect to being cold worked and annealed, these alloys resemble metals in general, in that they will recrystallize after having been cold worked if heated above the recrystallization temperature.

The term recrystallization, as understood in the art and as here employed, refers to the process by which new grains are formed from fragments of former grains in plastically deformed metal. As a result of recrystallization, the strains induced by the cold working are relieved. The temperature at which recrystallization begins varies with the degree of cold work and the rate of heating. The extent of recrystallization depends upon the temperature and the time at temperature. Whenever the conditions do not favor recrystallization, a substantial amount of the efiect of cold working is retained by the metal.

In making certain aluminum or aluminum base alloy articles from heavy sheet or plate, it sometimes becomes desirable to hot form the sheet or plate instead of shaping it at room temperature because of the inability or difiiculty of performing the shaping operations at room temperature. The hot forming operation involves a heating and shaping of the sheet or plate at a temperature preferably below, but sometimes above, the recrystallization temperature in order to utilize the advantage of greater workability associated with an elevated temperature. The softening effect obtained by heating for short periods below the recrystallization temperature exists only at that temperature and disappears on cooling and is to be distinguished from the permanent softening effect or annealing produced by complete recrystallization, since the latter softening efi'ect persists at room temperature after the heating. However, it is sometimes necessary to hot form cold worked alloys of the type herein-described at a temperature which ordinarily causes recrystallization and annealing to occur, and which results in a permanent softening with a considerable. loss of the strength as compared to the strength of the cold worked metal. In making many articles, it is preferred to use cold rolled material because of its higher strength and hard-' ness, as compared to material which has been hot worked or annealed. If advantage is to be taken of this increased strength and hardness, care must be exercised to avoid the softening of the material that accompanies the usual annealing treatment. It therefore becomes a problem in the hot forming process to secure an improved workability incident to the use of an elevated temperature on the one hand and on the other hand retain to a substantial extent, the properties of the cold worked metal.

The recrystallization temperature is also important in the joining of cold worked aluminum base alloys by brazing. The application of sufficient heat to the metal parts being joined causes a heating of the metal, with the resultthat recrystallization may occur in the cold worked metal.

Because cold worked copper-free aluminum base alloys containing from 0.25 to 10 per cent magnesium have proved to be so'successful for many purposes, it becomes desirable to extend their utility to other applications where fabricating difficulties have heretofore prevented the adoption of these alloys. My invention is directed to enlarging the temperature range within which the properties of these cold worked alloys can be retained to a substantial degree. A particular object of my invention is to provide a copperfree aluminum base alloy containing substantial amounts of magnesium which, after being cold worked, can be hot formed without losing the benefits of the col-d working. Another object is to provide an alloy of this character which can undergo the localized heating incident to brazing without undue decrease in strength of the metal in the neighborhood of the joint. A further object is to provide a material which is free from Liider lines when stretched or formed.

My invention resides in the discovery that the addition of from about 0.01 to 1 per cent zirconium to copper-free aluminum base alloys containing from 0.25 to 10 per cent magnesium serves to increase the recrystallization tempera ture of the cold Worked metal to a surprising extent as compared to the recrystallization temperature of the same composition without zirconium where both alloys have received the same work hardening and thermal treatments. I have found, for example, that by this means it is possible in some cases to raise the recrystallization temperature of an alloy by an amount which may be from 200 to 300 F. above that of the same alloy without zirconium. This eflect is of course modified by the degree to which the alloy has been work hardened and the duration of. the thermal treatment, the greatest effect being obtained where there has not been much work hardening and there is a short exposure to an elevated temperature. More specifically, I have observed in a number of tests that alloys of this kind do notshow any signs of recrystallization below 700 F., and in many cases recrystallization is still incomplete at 800 or 900 F. when heated for short periods of time. In other cases, recrystallization is not complete even though the alloy is held at the elevated temperature for as long as 24 hours. By thus raising the temperature of recrystallization, the temperature range is broadened within which these alloys may be heated without substantial loss in strength and hardness, as compared to their initial hardness and strength in the cold worked condition. It therefore becomes possible to heat the alloys up to the usual annealing temperature range of 600 to 700 F. without obtaining the permanent softening that is usually produced. Also, because of the elevation of the temperature of recrystallization, it is possible to carry out hot forming operations on these alloys at temperatures which are high enough to impart an increased workability without danger of softening through recrystallization.

Another advantage derived from the use of zirconium in the foregoing type of aluminum base alloys in addition to those mentioned above is that the appearance of Liider lines is prevented in drawn and formed articles. Liider lines are sets of wavy lines which sometimes appear on the surface of stretched metal but which have no effect upon the properties thereof. Because of the streaked appearance of the surface of the metal, articles exhibiting these lines are usually objectionable in commercial production since the lines can only be removed by a considerable amount of polishing.

Although zirconium is effective in the amount above stated, it is preferred to use from 0.1 to 0.5 per cent of this element, especially in alloys containing from 1 to 6 per cent magnesium.

The aluminum base alloys with which this invention is concerned, and which are termed copper-free, should be free from copper except as an impurity, since this element tends to reduce the resistance to corrosion of aluminummagnesium alloys. The copper impurity is preferably kept below 0.1 per cent.

Although aluminum-magnesium alloys are useful in and of themselves, it has been found advantageous at times to include certain metals known as hardening elements to enhance some desired properties. The metals which constitute this group of hardening elements and the amount of each metal to be used are as follows: manganese, 0.05 to 1.5 per cent; titanium, 0.01 to 0.5 per cent; chomium, 0.05 to 1.5 per cent; vanadium, 0.05 to 1 per cent; cobalt, 0.1 to 1 per cent; and nickel, 0.1 to 2 per cent. The total amount of these elements should not, however, exceed 4 per cent, and preferably less than 2 per cent. The addition of these elements does not destroy the effect of the zirconium and in some cases they may even accentuate the beneficial effect of zirconium in raisingthe temperature of recrystallization.

, The beneficial effect of adding zirconium to a from 0.25 to 10 per cent magnesium may be better appreciated through reference to the examples given hereinbelow. Four alloys were tested which had the following nominal compositions by weight:

(A) 1 per cent magnesium, 1.25 per cent manganese (B) 1 per cent magnesium, 1.25 per cent manganese, 0.25 per cent zirconium (C) 5.25 per cent magnesium, 0.1 per cent manganese, 0.1 per cent chromium (D) 5.25 per cent magnesium, 0.1 per cent manganese, 0.1 per cent chromium, 0.25 per cent zirconium.

The balance of the alloys in each case was a commercial grade of aluminum containing approximately 0.25 per cent of the usual impurities. Ingots of these alloys were hot rolled to slabs 0.25 inch in thickness in the usual manner and annealed, then cold rolled to sheet having a thickness of 0.064 inch, which represented a reduction of 75 per cent from the thickness of the previously annealed material. The cold rolled sheets were heated at 850 F. for 1 hour and cooled to room temperature to simulate a heating condition well above any which might be encountered in hot forming. Samples from these sheets were tested to determine their strength and elongation. The average values for'these properties were as follows; a

Tensile Yield Elonga- Alloy strength strength tlon Lbs. per Lbs. per

sq. in sq. in. Percent A 26, 4 11, 250 19. 20, 750 l2. 15, 21. 24, 900 24,

. ing from 0.25 to 10 per cent magnesium, from 0.01

to 1 per cent zirconium, and from 0.01 to 4 per cent of at least one hardening element of the group consisting of maganese 0.05 to 1.5 per cent, titanium 0.01 to 0.5 per cent, vanadium 0.05 to 1 per cent, cobalt 0.1 to 1 per cent, chromium 0.05 to 1.5 per cent, nickel 0.1 to 2.0 per cent, and the balance being aluminum, said alloy being characterized by having a higher recrystallization temperature than the same alloy devoid of zirconium where both alloys have received the same amount of work hardening and the same thermal treatment.

2. A cold worked aluminum base alloy consisting of from 0.25 to 10 per cent magnesium, 0.01 to 1 per cent zirconium, and the balance aluminum, said alloy lbeing characterized by having a higher recrystallization temperature than the same alloy devoid of zirconium where both alloys 3. A cold worked aluminum base alloy consisting of from 1 to 6 per cent magnesium, 0.1 to 0.5 per cent zirconium, and the balance aluminum, said alloy being characterized by having a higher recrystallization temperature than the same alloy devoid of zirconium where both alloys have received the same amount of work hardening and the same thermal treatment.

4. The method of producing a, cold worked aluminum base alloy containing between about 0.25 and 10 per cent magnesium and exhibiting improved recrystallization characteristics, comprising the steps, incorporating from about 0.01 to 1 per cent zirconium in the molten alloy at an appropriate stage in its production, casting a body of said alloy, cold working said alloy body, and thereafter heating the wrought body to a temperature below that at which complete recrystallization occurs but above the temperature at which a similar cold worked alloy devoid of 10 zirconium will completely recrystallize.

PHILIP T. STROUP. 

